Proteins AMINO ACID: STRUCTURE AND CLASSIFICATION. - PowerPoint Presentation

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Proteins

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AMINO ACID: STRUCTURE AND CLASSIFICATION.

 

Amino

Acids are the building units of proteins. There are about 300 amino acids occur in nature. Only 20 of them enter in proteins synthesis.

 

Structure of amino acids:

Four different

groups

are attached

to α-

carbon: amino group

COOH

group

Hydrogen atom

and

side Chain (R).

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At physiological pH (7.4), -COOH group is dissociated forming a negatively charged carboxylate ion (COO- ) and amino group is protonated forming positively charged ion (NH3 + ) forming

Zwitter

ion

.

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•

Proline

is an

imino acid not amino acid .

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Classification of Amino Acids:

I. Classification by R group

II. Chemical Classification

III. Nutritional Classification

IV. Metabolic Classification

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Classification by R group

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Classification according to polarity of side chain (R):

Polar amino acids

:

in which R contains polar hydrophilic group so can forms hydrogen bond with H2O. In those amino acids, R may contain:

OH group

: as in serine, threonine and tyrosine

- SH group

: as in cysteine

amide group

: as in glutamine and

aspargine

NH2 group or nitrogen

act as a base (basic amino acids ): as lysine, arginine and

histidine

COOH group

(acidic amino acids): as aspartic and glutamic .

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Non polar amino acids

: R is alkyl hydrophobic group which can’t enter in hydrogen bond formation

.

9 amino acids are non-polar ( glycine, alanine,

valine

,

leucine

, isoleucine, phenyl alanine, tryptophan,

proline

and methionine

).

 

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The

twenty common amino acids

are often referred to using three-letter abbreviations. The structures, names, and abbreviations for the twenty common amino acids are shown below. Note that they are all

α

-amino acids

.

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Each amino acid, aside from its name, has a three letter abbreviation and a one letter code.

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Nutritional Classification

1- Essential Amino Acids 10 in number, Can’t be synthesized in the body, essential to be taken in diet. Their deficiency affects growth, health and protein synthesis.

2- Semi-essential formed in the body but not in sufficient amount for body requirements especially in children. Arginine and

histidine

are semi-essential

.

3- Non-essential can be synthesized in the body.

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Non Standard Amino Acids

> 700 non standard amino acids have been detected in living organisms. Many are metabolic intermediates

eg

.

ornithine and

citrulline

are intermediates in urea

biosynthesis

Amino Acid Derivatives

Chemical derivatives of amino acids also have important biological functions,

eg

.

Catecholamines

(below) lack the a-carboxylate of amino acids

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GABA & Dopamine are neurotransmitters. Histamine mediates parts of the immune response.

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Functions of Amino Acids

Apart

from being the monomeric constituents of proteins and peptides, amino acids serve

variety

of functions

.

(a)

Some amino acids are converted to carbohydrates and are called as

glucogenic

amino acids

.

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(b)

Specific amino acids give rise to

specialised

products, e.g.•

Tyrsione

forms hormones such as

thyroid hormones

, (T3, T4),

epinephrine

and

norepinephrine

and a pigment called melanin.

• Tryptophan can synthesise a vitamin called niacin

.

•

Glycine, arginine and methionine

synthesise

creatine.

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• Glycine and cysteine help in

synthesis of Bile salts

.

•Glutamate, cysteine and glycine synthesis

glutathione

.

•

Histidine

changes to

histamine

on decarboxylation.

•

Serotonin is formed from tryptophan.• Glycine is used for the synthesis of haem..

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•

Pyrimidines

and purines use several amino acids for their synthesis such as aspartate and glutamine for

pyrimidines and glycine, aspartic acid, Glutamine and serine for purine

synthesis

c

)

Some amino acids such as glycine and cysteine are used as

detoxicants

of specific substances

.

(d)

Methionine acts as “active” methionine (S-adenosylmethionine) and transfers methyl group to various substances by transmethylation.(e)

Cystine

and methionine are sources of

sulphur